The present invention pertains to two-way hands-free devices, and more particularly to circuitry and methods for improving the operation thereof.
Bi-directional hands-free communication devices include devices such as two-way radios, speaker phones, commonly referred to as xe2x80x9chands-free telephonesxe2x80x9d, and teleconferencing devices and car-kits for cellular telephones, and the like. These hands-free communication devices include a speaker and a microphone, and therefore operation of such devices requires management of signals emitted by the speaker that are subsequently induced by the microphone. These signals, commonly referred to as xe2x80x9cecho signalsxe2x80x9d, are a nuisance to users and can in severe cases result in a phenomena known as xe2x80x9chowlingxe2x80x9d.
One known method of preventing echo signals is to allow only simplex, or one-way, communication to take place. Typically, simplex systems use a push to talk arrangement, wherein the speaker path is enabled and the microphone path is disabled. Only when the user operates a manual switch is the speaker path disabled and the microphone path enabled, allowing the user to talk to the remote device. Such systems prevent echo signals from developing, but are inconvenient since the user has to press the talk button each time they wish to talk. An additional problem associated with such systems is that the listening party can not interrupt the talking party, but rather must wait for the talking party to release their talk switch.
Echo suppressers and echo cancellers have evolved and are now well known devices for suppressing echo signals automatically. The need for a user to push a button before they talk can thus be eliminated. Echo suppressors automatically suppress the signal in one of the paths to prevent the total gain of both paths from rising above a threshold level. Typically, the first party to talk has the most gain, and the other party""s signal is suppressed until the first party stops talking. This type of operation is often referred to as half-duplex
Echo cancellers have been developed to provide improved performance, allowing a double talk condition to occur. Echo cancellers employ a filter to estimate the echo signal resulting from the speaker signal that is detected by the microphone. The echo canceller subtracts the echo signal estimate from the signal output by the microphone to produce an echo cancelled signal.
Although echo cancellers work well in some environments, the effective cancellation of echo signals in a hands-free vehicle environment is particularly challenging. Linear recursive filters, such as least means squares (LMS) error minimization, are often used for echo control. However, nonlinear and time-varying system effects, as well as limitations of algorithmic and arithmetic precision, limit the effectiveness of these echo cancellers. As a result, post processing stages are employed to suppress residual echoes. These post processing stages can include post processing procedures such as attenuation of the output signal through gain control or filtering, for example, or other known post signal processes.
However, post processing can result in significant degradation and attenuation of desired transmission signals that are present when both users are speaking simultaneously (double talk condition). The post processing attenuation results in half duplex characteristics, such that only one user can speak at a time. Additionally, post processing typically introduces perceptible changes, or attenuation of the background noise which is present in noisy environments, such as vehicle interiors. This noise variation correlates with speech activity in the signal received at the far end, such that it is objectionable to far-end users. In addition, hands-free units in varying noise environments, such as a vehicle interior, have fixed volume which can be too low or high for a given environment and require manual adjustment.
Accordingly, there is a need for improved control of a hands-free device to improve performance as perceived by both users of the device and remote users communicating with the hands-free device.